Printing system with self-purge, sediment prevention and fumes removal arrangements

ABSTRACT

A printing head assembly with integrated purge mechanism is disclosed. The printing head assembly comprises: (a) a liquid dispensing head comprising one or more dispensing nozzles enclosed in a nozzle plate, driven by at least first and second pressures, and (b) a shielding mask including an opening in front of the one or more nozzles, wherein the opening being configured such that when printing liquid is dispensed from the head driven by the first pressure, the liquid being dispensed in pulses through the opening in the shielding mask, and (ii) when purge printing liquid is dispensed from the head driven by the second pressure the liquid being drawn to a capillary gap formed between the shielding mask and the nozzle plate thereby removing the purge printing liquid from the nearby nozzles.

FIELD OF THE INVENTION

The present embodiment generally relates to the field of printing, andin particular, it concerns a printing system with an integratedself-purge, sediment prevention and fumes removal arrangements.

BACKGROUND OF THE INVENTION

It is known in the field of printing that inkjet printing heads requireperiodic purging to clean the printing nozzles, remove air bubbles, andmaintain printing quality. Simply stated, inkjet printers operate byexpelling a small volume of ink from a plurality of nozzles throughcorresponding small orifices in a nozzle plate held in proximity to apaper or front surface of a solar cell or other medium upon whichprinting or marks are to be placed. These orifices are arranged in afashion in the nozzle plate such that the expulsion of droplets of inkfrom a selected number of nozzles relative to a particular position ofthe medium results in the production of a portion of a desired characteror image. Controlled repositioning of the medium followed by anotherexpulsion of ink droplets results in the creation of more segments ofthe desired character or image.

A solar cell is a solid state semiconductor device that converts theenergy of sunlight directly into electricity. Most large-scalecommercial solar cell factories make printed poly-crystalline siliconsolar cells. The printed poly-crystalline silicon solar cells havegrid-like metal contacts made up of fine fingers and larger bus barsprinted onto the front surface using a silver paste ink jet printing.The grid-like metal contact is made of 60 to 100 printed silver lines, 2millimeter (mm) apart and having a typical width of 100 micrometers(um).

The frequency of purging the printing head depends on the specificapplication for which the printing head is being used. Purging includesforcefully pushing a printing liquid out of the inkjet head through thenozzles under an excess pressure of roughly 0.5 Bar. The force of theexcess pressure is relative to a normal reduced pressure used duringprinting of about −0.01 Bar.

One of the disadvantages of conventional purging techniques is that aprinting head needs to be shifted from the printing area to amaintenance area where the purged liquid is able to drip from the head,and any liquid that drips from the head will not adversely affect theprinting process. A nozzle plate, as is generally known in the industry,is located on the printing side of the printing head, providing accessfor the nozzles to print while providing protection for the printinghead, among other features.

After purging, it is preferable for a nozzle plate to be cleaned, knownas wiping, to remove purged liquid and enable proper jetting of theprinting liquid from the nozzles. In order to preserve the smoothnessand anti-wetting characteristic of the nozzle plate, it is desirable toperform wiping without contact to the nozzle plate.

One conventional technique for wiping without contact to the nozzleplate is vacuum wiping, where a vacuum head is moved across the nozzleplate. The vacuum head does not contact the nozzle plate but issufficiently close to allow the vacuum, also known as suction, to removethe purged liquid from the nozzle plate. As the vacuum head does notcontact the nozzle plate, there is suction from all sides of the vacuumhead (not just from the direction of the nozzle plate) resulting in lowcleaning efficiency of the nozzle plate.

Disadvantages to conventional vacuum wiping include cost, printingspeed, reliability, and quality of wiping. Examples of the disadvantagesof conventional vacuum wiping include: (a) The printing head requiresshifting from the printing area to a maintenance area due to thepossibility of purged liquid dripping from the printing head. (b)Shifting a printing head from a printing area to a maintenance areatakes time, resulting in reduced printing speed and correspondinglyincreased cost.(c) The printing machine requires very high precisionelements and structure to enable a vacuum slit to move across the nozzleplate at a small distance without contact, for example about 0.15 mm.Higher precision elements increase the cost of a printing machine, andthe reliability of wiping is dependent on maintaining high precision.(d) Effective wiping requires the vacuum slit to move across the nozzleplate at a slow speed, for example about 2 mm/s (millimeters persecond). The time required for effective wiping reduces printing speedand correspondingly increases printing cost.

There is therefore a need for a printing system that may be used forprinting a solar cell grid-like metal contact for example that enablespurging without purged liquid dripping from the nozzle plate (or ingeneral from the printing head). It is further desirable for highquality cleaning with lower costs and higher reliability than currenttechniques. Furthermore, there is a need to prevent the formation ofsediment on the nozzle plate and for removing part of the fumesaccumulated on the nozzle surface to prevent deflection of the printedink jets.

SUMMARY OF THE INVENTION

Embodiments of the present invention disclose a printing head assemblywith integrated purge mechanism. The printing head assembly comprises:(a) a liquid dispensing head comprising one or more dispensing nozzlesenclosed in a nozzle plate, driven by at least first and secondpressures, and (b) a shielding mask including an opening in front of theone or more nozzles, wherein the opening being configured such that whenprinting liquid is dispensed from the head driven by the first pressure,the liquid being dispensed in pulses through the opening in theshielding mask, and (ii) when purge printing liquid is dispensed fromthe head driven by the second pressure, the printing liquid being drawnto a capillary gap formed between the shielding mask and the nozzleplate thereby removing the purge printing liquid from the nearby nozzle.

According to a further feature of an embodiment of the presentinvention, the printing head assembly includes one or more printing headassemblies with integrated purge mechanisms.

According to a further feature of an embodiment of the presentinvention, the printing head assembly includes a plurality of printingheads, wherein the shielding mask includes an opening in front of eachof the printing heads.

According to a further feature of an embodiment of the presentinvention, the shielding mask opening includes a slit.

According to a further feature of an embodiment of the presentinvention, the one or more nozzles are configured close to an edge ofthe slit in each of the printing heads such that the purge printingliquid is drawn to the capillary gap via the edge of the slit.

According to a further feature of an embodiment of the presentinvention, the slit is off center relative to the nozzles such that adistance from the nozzles to one edge of the slit is less than 20% of adistance from the nozzles to an opposing edge of the slit.

According to a further feature of an embodiment of the presentinvention, a suction system is in fluid interconnection with thecapillary gap, the suction system providing a pressure gradient to thecapillary gap, thereby removing the purge printing liquid from thecapillary gap.

According to a further feature of an embodiment of the presentinvention, a suction system is in fluid interconnection with thecapillary gap removably attached underneath the opening, the suctionsystem providing a pressure gradient to the opening, thereby removingthe purge printing liquid from the capillary gap via the opening.

According to a further feature of an embodiment of the presentinvention, the shielding mask further includes a printing head housing.

According to a further feature of an embodiment of the presentinvention, the printing head assembly includes a cooling system thatcools the printing head housing.

According to a further feature of an embodiment of the presentinvention, the printing head assembly includes further a printing liquidstorage and re-circulation system that returns the purge printing liquidthat has been captured in the capillary gap to the printing headassembly printing liquid storage.

According to a further feature of an embodiment of the presentinvention, the purge printing liquid may be dispensed while the printinghead is in a printing area.

According to a further feature of an embodiment of the presentinvention, the printing head housing further includes an auxiliarysuction system deployed to evacuate at least part of the fumes thatemerge from the dispensed printing liquid on a heated printed substrate,or from a hot pool of volatile liquid underneath the printing head whilethe printing head exits the printing area, to maintain a predefinedlevel of moisture adjacent to the nozzles plate.

According to a further feature of an embodiment of the presentinvention, a method for printing is disclosed. The method includes thesteps of: (a) providing a printing head assembly with integrated purgemechanism, each printing head assembly including:(i) a printing headincluding one or more nozzles; and (ii) a shielding mask including anopening aligned with the multitude of nozzles; wherein a capillary gapis created between the printing head and the printing mask; (b) printingby actuating the printing head with a first pressure such that aprinting liquid is dispensed in pulses from the printing head passingthrough the opening; and (c) purging by forcing printing liquid throughthe one or more nozzles with a second pressure such that the dispensedliquid is drawn to the capillary gap.

According to a further feature of an embodiment of the presentinvention, the method uses one or more printing head assemblies withintegrated purge mechanisms.

According to a further feature of an embodiment of the presentinvention, the shielding mask including an opening in front of each ofthe printing heads.

According to a further feature of an embodiment of the presentinvention, the shielding mask opening includes a slit.

According to a further feature of an embodiment of the presentinvention, the method includes the step of configuring the one or morenozzles close to an edge of the slit in each of the printing heads suchthat the purge printing liquid is drawn to the capillary gap via theedge of the slit.

According to a further feature of an embodiment of the presentinvention, the method includes the step of configuring the slit offcenter relative to the nozzles such that a distance from the nozzles toone edge of the slit is less than 20% of a distance from the nozzles toan opposing edge of the slit.

According to a further feature of an embodiment of the presentinvention, the method includes the step of re-circulating the printingliquid that has been drawn to the capillary gap to a printing headassembly printing liquid storage.

According to a further feature of an embodiment of the presentinvention, the method includes the step of removing the printing liquidfrom the capillary gap using a suction system fluidly connected to thecapillary gap, the vacuum system providing a pressure gradient to thecapillary gap, thereby removing the printing liquid from the capillarygap.

According to a further feature of an embodiment of the presentinvention, the method includes the step of removing the printing liquidfrom the capillary gap using a suction system in fluid interconnectionwith the capillary gap removably attached to the underneath the opening,the suction system providing a pressure gradient to the opening, therebyremoving the purge printing liquid from the capillary gap via theopening.

According to a further feature of an embodiment of the presentinvention, a method for printing by an inkjet head is disclosed. Themethod includes jetting nozzles enclosed in a nozzles plate, wherein thenozzles plate being kept wet during printing so as to preventaccumulation of solid sediment on the nozzles plate.

According to a further feature of an embodiment of the presentinvention, the step of keeping the nozzles plate wet is accomplished byfumes that emerge from the ejected printing liquid accumulated on aheated substrate.

According to a further feature of an embodiment of the presentinvention, the step of keeping the nozzles plate wet is accomplished byfumes that emerge from a hot pool of volatile liquid underneath theprinting head while the printing head exits the printing area.

According to a further feature of an embodiment of the presentinvention, the step of removing part of the fumes by an auxiliarysuction system is performed in order to maintain a predefined level ofmoisture adjacent to the nozzles plate.

According to a further feature of an embodiment of the presentinvention, the step of wiping the nozzle plate intermittently to controlthe size of the wetting droplets on the nozzles plate.

According to a further feature of an embodiment of the presentinvention, the step of shielding the printing inkjet head partially fromthe printed substrate's heat and fumes using a shielding mask includingan opening to maintain a predefined level of moisture adjacent to saidnozzles plate.

According to a further feature of an embodiment of the presentinvention, the amount of fumes spread near the jetting nozzles iscontrolled by installing the nozzles close to an edge of the openingthrough the shielding mask.

According to a further feature of an embodiment of the presentinvention, a purge sequence of the printing inkjet head cleans up thespread of liquid droplets on the nozzles plate.

Additional features and advantages of the invention will become apparentfrom the following drawings and description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect, reference will now be made, purely by way ofexample, to the accompanying drawings in which like numerals designatecorresponding elements or sections throughout.

With specific reference now to the drawings in detail, it is stressedthat the particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention, the description taken with the drawings making apparent tothose skilled in the art how the several forms of the invention may beembodied in practice. In the accompanying drawings:

FIG. 1A illustrates a printing system with an integrated self-purgearrangement according to an embodiment of the present invention;

FIG. 1B illustrates a second view of a printing system with anintegrated self-purge arrangement according to an embodiment of thepresent invention;

FIG. 1C illustrates a third view of a printing system with an integratedself-purge arrangement from the direction in which the ink is printedaccording to an embodiment of the present invention;

FIG. 2A illustrates a front view of a vacuum pipe system for clearingpurged liquid, according to an embodiment of the present invention;

FIG. 2B illustrates a side view of a vacuum pipe system for clearingpurged liquid, according to an embodiment of the present invention;

FIG. 3 illustrates a mechanism for clearing purged liquid, according toan embodiment of the present invention;

FIG. 4 illustrates a detailed diagram of a mechanism for clearing purgedliquid, according to an embodiment of the present invention;

FIG. 5 illustrates the printing system capillary gap and evacuation ofthe purge printing liquid, according to an embodiment of the presentinvention;

FIG. 6A illustrates a mechanism for removing fumes from the gap betweenthe printing head and the printing substrate according to an embodimentof the present invention;

FIG. 6B illustrates a mechanism for removing part of the fumes from thegap between the printing head and the printing substrate according to anembodiment of the present invention;

FIG. 7 illustrates fumes wetting the printing head nozzle plateaccording to an embodiment of the present invention;

FIG. 8 illustrates tiny droplets formed on the printing head nozzleplate according to an embodiment of the present invention;

FIG. 9 illustrates large droplets that may disturb ejection of inkthrough the nozzles according to an embodiment of the present invention;

FIG. 10 illustrates an ink droplet floating over tiny droplets accordingto an embodiment of the present invention;

FIG. 11 illustrates an evaporation bath as a source of fumes accordingto an embodiment of the present invention;

FIG. 12 illustrates an nozzle located close to the edge of the slitaccording to an embodiment of the present invention;

FIG. 13 illustrates a bottom view of the nozzle plate slit according toan embodiment of the present invention;

FIG. 14 illustrates a printing head assembly with an integratedself-purge and fumes removal arrangements according to an embodiment ofthe present invention;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention provide a printing system with anintegrated self-purge, sediment prevention and fumes removalarrangements without purged liquid dripping from the nozzle plate. Theintegrated self-purge arrangement is desirable for high quality cleaningwith lower costs and higher reliability than current techniques. Inparticular, the present embodiment facilitates printing whileeliminating the time required for shifting the printing head to amaintenance area, as compared to the time required for conventionaltechniques. In addition, in a multi-head system (or a system withmultiple groups of heads) where one maintenance area (or in generalfewer maintenance areas than heads) are used for purging, the presentembodiment facilitates reduced system complexity. For example, each head(or group of heads) has an integrated self-purge arrangement eliminatingthe need for competition and coordination of heads for maintenanceareas. Although this embodiment is described with regard to an inkjetprinting head, the described system and method is generally applicableto liquid-ejection nozzles of a liquid-ejection mechanism, such asnozzle dispensers.

In the context of this document, the terms printing liquid and ink referin general to a material used for printing, and includes, but is notlimited to homogeneous and non-homogenous materials, for example acarrier liquid containing metal particles to be deposited via theprinting process.

According to a first aspect of the present invention, a printing headassembly with integrated purge mechanism, including: (a) a liquiddispensing head comprising one or more dispensing nozzles enclosed in anozzle plate, driven by at least first and second pressures, and (b) ashielding mask including an opening in front of the one or more nozzles,wherein the opening being configured such that when printing liquid isdispensed from the head driven by the first pressure, the liquid beingdispensed in pulses through the opening in the shielding mask, and whenpurge printing liquid is dispensed from the head driven by the secondpressure, the liquid being drawn to a capillary gap formed between theshielding mask and the nozzle plate thereby removing the purge printingliquid from the nearby nozzles.

According to embodiments of the present invention, capillary action isused to capture printing liquid and prevent dripping of printing liquidfrom the printing head to the printing substrate

Capillary action is the tendency of a liquid to be drawn into smallopenings. Capillary action, also known as capillarity, is a result ofthe intermolecular attraction within the liquid and solid materials. Afamiliar example of capillary action is the tendency of a dry papertowel to absorb a liquid by drawing the liquid into the narrow openingsbetween the fibers. Another example is the tendency of liquids to risein narrow tubes. The mutual attractive force that exists between likemolecules of a particular liquid is called cohesion. Cohesion producesthe phenomenon known as surface tension. When an attractive force existsbetween two unlike materials, for example a liquid and the edges of asolid container, the attractive force is known as adhesion. Capillaryaction is the result of cohesion of liquid molecules and adhesion ofthose molecules to the solid material forming a void, for example a voidformed by the walls of a narrow tube. As the edges of the container arebrought closer together, for example in a very narrow tube, theinteraction of cohesion and adhesion causes the liquid to be drawn intothe void formed by the edges of the container, in the current exampleinto the narrow tube. In the context of this document, the void isformed by the capillary gap between the nozzle and mask plates.

A further aspect of the present invention relates to methods forpreventing sediment accumulation on the printing head nozzles plate thatmay block or deflect the ink jets using fumes and for removing part ofthe fumes accumulated underneath the printing head on the nozzles plate.

According to a further aspect of the present invention, certainembodiments provide a method for printing with an integrated self-purge,sediment prevention and fumes removal arrangements. The methodcomprising the steps of (a) providing a printing system comprising oneor more printing head assemblies with integrated purge mechanisms, eachprinting head assembly comprising: a printing head including one or morenozzles and a printing mask including an nozzle plate with an opening,aligned such that the nozzle plate opening is aligned with the one ormore nozzles and a capillary gap is created between the printing headand the printing mask. (b) printing by actuating the printing head witha first pressure such that a printing liquid is ejected from theprinting head passing through the nozzle plate opening, and (c) purgingby actuating the printing head with a second pressure such that theprinting liquid that has been ejected through one or more nozzles viathe second pressure is captured by the capillary gap.

According to a further aspect of the present invention, the printinghead is shielded from the substrate's heat and fumes by a mask, whereinthe mask comprising a slit through which the ink is dispensed on thesubstrate. Furthermore, the printing head nozzles are located close toone edge of the slit and hence the major part of the fumes that enterthe slit condensate at the center area of the nozzle plate as seenthrough the slit, and only a minute part of the fumes condensate nearthe edge of the slit.

The various aspects of the invention will be illustrated hereinprimarily by reference to a non-limiting example of a system and methodfor printing solar cells grid like contacts, but it will be appreciatedthat the various aspects of the present invention are equally applicableto a wide range of other ink jet printing systems.

FIG. 1A illustrates a first view of a printing system with an integratedself-purge arrangement according to an embodiment of the presentinvention. For convenience FIG. 1A and FIG. 1B are arbitrarily referredto as a front view and side view, respectively. Note that figures arenot drawn to scale. An inkjet printing head 100 includes an nozzle plate102. Ink is printed from a plurality of nozzles (not shown) in thedirection of arrows 108 to a printing substrate (not shown) that may bea front surface of poly silicon solar cell. Note that this system can beused for one or more nozzles, although normal usage in this field iswith multiple number of nozzles. For convenience, the direction of theink from the printing head to the printing substrate shown by arrows 108is referred to as downward. FIG. 1A shows a plurality of arrows 108indicating the printing direction of ink from an array of nozzles, whilethe side view of FIG. 1B shows only one arrow as from a side view onlythe single array is visible. A feature of this implementation is thatthe positioning of a printing mask 104, also referred to in thisdocument as a mask, aligned with nozzle plate 102 creates a capillarygap 110 between the nozzle plate and the printing mask. During purging,the purged liquid adheres to the surfaces of capillary gap 110 andcapillary action draws the purged liquid into the capillary gap. Thenozzles of the printing head are aligned with a slit 106 in the printingmask 104 to facilitate printing.

FIG. 1C illustrates a third view of a printing system with an integratedself-purge arrangement from the direction in which the ink is printed.For convenience, FIG. 1C is also referred to as a bottom view. Forreference, the slit 106 has width 112 and length 114. Height 116, alsoreferred to as depth, is generally substantially the same as thethickness of the printing mask.

A printing mask 104 is aligned with an nozzle plate 102 and positionedto create a capillary gap 110 between the printing mask and the nozzleplate. In the context of this document, a mask refers to a plate thatpartially covers nozzle plate 102 and has an opening to facilitateprinting from nozzles to a print area. A nozzle plate 102 is generallyused during the printing process to facilitate printing from the nozzlesand can also provide protection for the printing head 100 and nozzles.In normal operation slit 106 in printing mask 104 is sufficiently wideand aligned sufficiently accurately with the printing nozzles tofacilitate printing. In the case of an inkjet printing head 100,printing includes jetting droplets of ink from nozzles (not shown) inpulses where the jetting pressure is created inside the printing head100 by piezoelectric crystals, one piezoelectric crystal for eachnozzle. Jetting includes applying an appropriate pressure for anappropriate duration to the printing head, causing the printing head todischarge droplets of a printing liquid (ink) from the nozzles, throughan opening (not shown) in nozzle plate 102, across gap 110, through slit106 in printing mask 104, and onto a printing substrate (not shown). Inone non-limiting example, a 20 um wide nozzle prints through a slithaving a width 112 between 100 and 300 um.

During purging, an appropriate extra pressure is applied externally tothe printing liquid and through the printing head 100 for an appropriateduration to push forcefully ink out of the printing head through thenozzles. The ink that emerges from the nozzles during purging firstattaches to the nozzle plate 102 due to adhesion (as defined below, anattractive force between two unlike materials), which in this case isalso known as the ink's tendency to wet the nozzle plate. Wetting of thenozzle plate occurs even when the nozzle plate is coated with ananti-wetting coating. When mask 104 is aligned sufficiently close tonozzle plate 102, the mask is also wet by the ink. Due to adhesion, theink does not tend to pass through slit 106. If mask 104 is positionedappropriately with respect to nozzle plate 102, the purged ink will tendto be drawn into capillary gap 110 due to capillary forces. The edges ofcapillary gap 110 share at least a portion of an edge of slit 106.Colloquially, because the purging does not require any additionalhardware below the mask 104 (for collecting ink that may drip duringpurging and wiping the nozzle plate) this technique is referred to as“hidden purge”.

The required size of a capillary gap to create a desired capillaryaction depends on the specific application, and more specifically, onthe properties of the surfaces themselves and the liquid. In the contextof this document, references to the size of the capillary gap refer tothe distance between nozzle plate 102 and mask 104, unless otherwisespecified. Given the physical characteristics of a printing liquid,nozzle plate, and mask plate, one skilled in the art can calculate therequired size of a capillary gap to provide a desired capillary action.

In a non-limiting example where the nozzles are 20 um wide, thecapillary gap 110 may be as large as 50 to 500 um. Within the bounds ofthe above-described embodiment, the preferred size for the capillary gap110 is to be as small as possible so the nozzles can be as close aspossible to the printing substrate, facilitating higher qualityprinting. Similarly, to enhance capillary action, the capillary gapshould be as small as possible. The minimum capillary gap size dependsat least in part on the physical characteristics of a printing liquid.The capillary gap size must be large enough to facilitate the flow andcollection of printing liquid for the specific application. Tofacilitate capillary action, the purged printing liquid shouldpreferably adhere to at least one of the slit edges (gap edges), hencethe nozzles should be positioned as close as possible to a slit edge,facilitating the purged ink adhering to at least one slit edge (edges ofthe capillary gap). In the present example, a distance from a nozzle toa slit edge of 50 to 300 um is appropriate.

It is preferable for the slit to be sufficiently large to facilitateoccasional wiping of the nozzle plate, for example, by a resilient thinwiper. In the current example, a slit width of 0.7 to 1.5 mm issufficient. In this case, it is preferable for the nozzles to bepositioned closer to an edge of the slit (in contrast to beingpositioned in the middle of the slit), to facilitate purged printingliquid adhering to the edge of the slit.

Similarly, mask 104 needs to be sufficiently thick (dimension 116) toprovide the necessary mechanical strength and heat conduction, andpreferably as thin as possible so the nozzles can be as close aspossible to the printing surface. After purging, a delay providessufficient time for substantially all of the purged liquid to flow fromnozzle plate 102 into capillary gap 110. In a non-limiting example,after purging an inkjet head a delay of between 2 to 8 seconds issufficient for the nozzle plate to be clear of ink, providing a highquality cleaning. After the delay, the printing head is ready to be usedfor printing.

It is preferable that both the nozzle plate and the inner side of theprinting mask (in this context, the side of the printing mask that facesthe nozzle plate) are coated with an anti-wetting coating. Ananti-wetting coating reduces the tendency of the printing liquid toadhere to the surfaces of the nozzle plate and the inner side of theprinting mask. The purged printing liquid needs to be removed from thecapillary gap, or in other words cleared from the capillary gap. Thefrequency of clearing the capillary gap depends on the application. Inone case, the capillary gap is cleared after every purge. Theimplementation of a clearing system depends on the application.

FIG. 2A and FIG. 2B illustrate a front view and a side view,respectively, of a vacuum pipe system for clearing purged liquid, avacuum pipe system 200 is shown below mask 104 covering a portion ofslit 106. In this case, the vacuum pipe system 200 includes a vacuumhead 202 that moves along slit 106 from below. Vacuum head 202 connectsto, or is part of, tube 204 which is connected to a pump (not shown) Avacuum, for example a reduced pressure of −0.4 to −0.8 Bar, is appliedto tube 204 by the pump, and the force of the pressure gradient suctionsthe purged liquid from capillary gap 110 through slit 106 in thedirection of arrow 206.

Vacuum head 202 can be firmly attached to the floor of the mask 104covering the slit 106 so the suction of the vacuum head is primarilyfrom the slit, while the vacuum head does not contact the nozzle plate.As can be seen in FIG. 2B, vacuum head 202 contacts mask 104 and coversthe width of slit 106. In contrast, in the above-described conventionalvacuum wiping technique, the vacuum head does not contact the nozzleplate resulting in low cleaning efficiency of the nozzle plate. In thepresent embodiment, cleaning the nozzle plate 102 is done during orafter purge when the purged ink is drawn into the gap by capillaryaction with a high cleaning efficiency, and the vacuum pipe system 200evacuates the purged liquid from the capillary gap 110 via the slit 106.In another embodiment, cleaning the nozzle plate 102 can be done duringor after purge, by the vacuum pipe system 200 evacuating the purgedliquid from the nozzle plate via the slit. Additionally cleaning time isreduced, as compared to conventional vacuum wiping. Vacuum pipe systemsare known in the art, and based on this description one skilled in theart will be able to implement a vacuum pipe system appropriate for theapplication.

The above-described method is effective. However, in certain casesclearing a capillary gap without a system element below the printingmask is preferable. This feature is preferable in cases where the headis above the printing surface during cleaning, and facilitates reducingthe time required for maintenance, hence increasing the printing speed.

FIG. 3 illustrates a mechanism for clearing purged liquid, the printinghead further includes a printing head housing 300, also simply known asa housing, which partially encloses printing head 100. Note that in theart a print head housing 300 is also sometimes referred to as a “mask”,but should not be confused with mask 104, as used in this document.Housing 300 includes a side portion 302 that surrounds the sides ofprinting head 100. A bottom portion 304, also known as the floor, ofhousing 300 functions as mask 104 and partially encloses nozzle plate102. Housing 300 includes one or more suction ports 306 connected to avacuum system 310. The suction ports 306 facilitate the purged liquidbeing suctioned from capillary gap 110 out of the housing. Vacuum system310 can be implemented similarly to the above-described vacuum pipesystem 200, without requiring movement of the vacuum pipe system vacuumhead 202. Vacuum system 310 can remain connected to housing 300.

In one non-limiting example, the system performs a purge followed byoperation of vacuum system 310. In this case, the purged liquid is drawninto the capillary gap, as described above. At a determined time afterthe purge is completed, the vacuum system 310 is operated, evacuatingthe purged liquid out of the housing, as described above. In anothernon-limiting example, the vacuum system 310 is operated during purge. Inthis case, the purge is more efficient in that the suction addsadditional pressure to the capillary action, helping to prevent purgedliquid from dripping, and facilitates a faster purge, thus reducingcleaning time as compared to the above-described techniques. Incontrast, in conventional purging techniques the vacuum wiping cannot beperformed during purge, because purging is done from all nozzles at thesame time (parallel nozzle purging), while the vacuum wiping is done bya vacuum head that only covers a portion of the nozzles at a time as thevacuum head moves across the nozzle plate. In an optional embodiment,the vacuum system 310 can operate independent of capillary action,during or after purge, evacuating the purged liquid from the nozzleplate via the gap 110. Optionally the purged liquid can be recycled fromthe vacuum system via appropriate filtering systems to the ink supplysystem. Vacuum systems are known in the art, and based on thisdescription one skilled in the art will be able to implement a vacuumsystem 310 appropriate for housing 300 and the application.

FIG. 4 illustrates a detailed mechanism for clearing purged liquid, thehousing 300 further includes a cooling channel 400 through which acoolant passes. Depending on the application, printing head 100 and/orhousing 300 may get hot from printing and require cooling, for exampleto protect the system from damage and/or maintain printing quality.Commonly, a heat source is outside the housing 300, and the housingprotects the printing head 100 from damage due to heat. In anon-limiting example, main heat source is the printed substrateunderneath the housing 300, such as solar cells silicon wafers on whichmetal lines are being printed. The solar cells silicon wafer may enterthe printing zone after being warmed to 250 degrees Celsius. Thistemperature may be required to evaporate a liquid carrier of the metalparticles including the ink such as silver particles. In this case, thehousing 300 functions as a heat barrier that prevents the heat from thesolar cells wafer from reaching the nozzle plate 102 of the printinghead 100. In one optional embodiment, cooling channel 400 is a channelfor a liquid coolant. The liquid coolant circulates through the channelas appropriate for the application, providing cooling for the system.Housing 300 can be constructed from aluminum, copper, or anothermaterial appropriate for the application, preferably a material that isa good heat conductor.

In a non-limiting example, housing 300 is constructed of aluminum andprinting head 100 is an inkjet head. Referring also to FIGS. 1A, 1B, and1C, slit 106 has width 112 of 0.3 millimeter (mm). The slit width can besmaller or larger depending on the application. It is preferable thatthe slit width is larger than the diameter of the jetted droplets, andadditionally large enough to allow for inaccuracies in straightness andalignment of nozzles. In a non-limiting example, when droplet diameteris 20 um, a slit width of 50 um is a preferred a minimum width. In acase where the slit is larger than the preferred minimum, it ispreferable that the nozzles are sufficiently close to an edge (side) ofthe slit in order to facilitate the purged liquid adhering to the edgesof the capillary gap 110. A non-limiting example of this case is where aslit width of 1 to 2 mm is used instead of a minimum slit width of 0.3mm, and the nozzles are aligned to be 0.1 mm from an edge of the slit.As described above, once purged liquid adheres to an edge of thecapillary gap 110, capillary action draws the purged liquid into thecapillary gap behind the mask 104. Note that it is generally sufficientto draw the purged liquid into the capillary gap on one side of theslit, in this case, the side closer to which the nozzles have beenaligned. In a case with an array of nozzles that includes two rows ofnozzles, the slit width and alignment can be designed so that each rowof nozzles is close to a respective edge of the slit. A height 116 ofthe slit of 0.3 mm corresponds to the thickness of the mask and isimplemented as floor 304 of housing 300. The height can be smaller orlarger depending on the application. While a smaller height, for example0.2 mm, is possible, a smaller height is limited by practicalconsiderations. Practical considerations include, but are not limitedto, a smaller height corresponds to a thinner mask that generally hasinsufficient heat conductivity and is more easily damaged. While alarger height, for example 1 mm, is also possible, a larger height islimited by practical considerations including, but are not limited to,requiring a greater separation between a printing head and printingsubstrate, which as described above tends to reduce printing quality. Acapillary gap 110 of 0.3 mm between the nozzle plate and the mask ispreferable, but depending on the application, the capillary gap can befrom 0.05 mm to about 0.5 mm. A spacing 410 between side portion 302 ofthe housing and printing head 100 is preferably 0.3 mm, but depending onthe application, the capillary gap can be from 0.1 mm to 1 mm.

Arrows 402 show the direction of flow of the purged liquid. Note thatalthough in the two-dimensional diagram of FIG. 4 the arrows show flowaway from slit 106 toward the left and right, in an actualthree-dimensional system the flow is outward from slit 106 towardsuction ports 306 connected to a vacuum system 310. Clearing can beinitiated by activation of vacuum system 310 optional valve V, or othermeans appropriate for the application. In an alternative embodimentduring purging the printing liquid that emerges from the nozzlesattaches to the nozzle plate. A vacuum system 310 provides a negativepressure via gap 110 and suction cleans the purged printing liquid fromnozzle plate 102.

FIG. 5 illustrates the printing system capillary gap and evacuation ofthe purging liquid, according to an embodiment of the present invention.Printing head 100 and housing 300 with optional coolant 400 areillustrated. Purge liquid 510 is driven through the array of nozzles(not shown) and through nozzle plate 102. The purge liquid 510 is drawnto capillary gap 110 and evacuated by vacuum pipe system through thecapillary gap 110 and out of the printing head housing 520. The purgeliquid may be re-circulated and stored in the interior portion of theprinting heads configured to contain printing liquid.

The nozzle plate 102 of inkjet heads 100 is often coated by anon-wetting material that repels the printing liquid and prevents theprinting liquid from sticking to the plate. As a consequence, the liquidthat flows on the nozzle plate 102 easily converts to large drops 510that are lightly attach to the nozzle plate and can be removed from itby a by the capillary gap 110 and by a vacuum pipe system.

Ink may accumulate on nozzle plate 102 in several ways: 1. Duringpurging, the ink continuously flows out of the nozzle, and even with thebest non-wetting coating the ink spreads over the nozzle plate ratherthan dropping off into the open air and substrate. 2. Jetted satellites(i.e. extremely small parasite droplets that accompany the much largermain droplet comprising a jetted ink) lose their velocity immediatelyafter emerging from the nozzle (because of friction with air) and caneasily flow back in the air and land on the nozzle plate. 3. Some minutepart of the jetted ink that impinges on the substrate might go back likea ricochet towards the nozzle plate and stick to it. 4. Particularlywhen printing on a heated substrate, the solvent of the printing fluiddeposited on the substrate evaporates rapidly, generating fumes ofsolvent vapor which may condense on the colder nozzle plate 102.

Particularly when printing with printing liquid (“ink”) which containssolid particles, such as in the particularly preferred but non-limitingexample of printing with conductive metal particles such as silver, thepresence of ink on the nozzle plate may lead to significant problems.Specifically, if the ink on the nozzle plate is allowed to dry, a layerof particles is deposited onto the surface. Over time, multiple suchlayers may accumulate. These layers degrade the non-wetting surfaceproperties of the nozzle plate and, if built up sufficiently close tothe nozzles, may directly reduce print quality and eventually completelyclog the nozzles.

According to a further aspect of the present invention, such depositionof particles is eliminated or reduced by managing the presence of liquidvapor in proximity to the nozzle plate so that any ink present on thenozzle plate does not dry on. The still-wet ink can then be removedperiodically, by mechanical wiping or by suction, to prevent build up ofmaterial on the nozzle plate. Particularly in the preferred case ofprinting on a high-temperature substrate, such as a heated semiconductorwafer, solvent vapor is inherently generated from ink deposited on thesubstrate. However, excess vapor causes accumulation of large dropletson the nozzle plate which may themselves interfere directly with theprinting process. Accordingly, this aspect of the present inventionprovides a vapor (or “fumes”) management system which removes excessvapor while leaving sufficient vapor in the proximity of the nozzleplate to ensure that any ink on the surface does not dry out, yetwithout forming large droplets.

Assuming that the printing head comprises an array of nozzles, there aretwo ways to control the amount of fumes: evacuating part of the fumes byan auxiliary vacuum opening before the fumes reach the nozzle area andthe second way refers to a case where in head is shielded from thesubstrate's heat and fumes by a mask, wherein the mask comprising a slitthrough which the ink is dispensed on the substrate. In such cases mostof the fumes that enter the slit condense at the center area of thenozzle plate as seen through the slit, and only a lesser quantity of thefumes condense near the edge of the slit. In order to take advantage ofthis phenomenon, according to certain preferred implementations of thepresent invention, the slit in the mask is located such that the nozzlesof the printing head are close to one edge of the slit. This positioningof the nozzles adjacent to one edge of the slit is also advantageous forthe aforementioned capillary take-up of printing fluid during the purgeprocess, ensuring that droplets forming at the nozzles during the purgeprocess quickly come into contact with the capillary gap and are drawninto the gap.

FIG. 6A illustrates a mechanism for removing excess fumes from the gapbetween the printing head and the printing substrate according to anembodiment of the present invention. Fumes 610 that may be formed by ahot substrate 620 after the printing liquid is ejected by printing head100 through nozzles in the nozzle plate 102 are removed by auxiliarysuction system 640 through the printing head housing 300.

FIG. 6B illustrates a mechanism for removing part of the fumes from thegap between the printing head and the printing substrate according to anembodiment of the present invention. Part of the fumes 610, typically90% of the fumes, are being evacuated by auxiliary suction system 640and the remaining part of the fumes 615 are accumulating on the nozzleplate 102. With very low fumes pressure the nozzle plate will be coveredgradually by ink droplets and hence by solid sediment included in theink such as silver particles for example. With high fumes pressureprinting liquid droplets will agglomerate and may deflect and block thenozzle plate 102 nozzles.

FIG. 7 illustrates fumes wetting the printing head nozzle surfaceaccording to an embodiment of the present invention. A proportion of thefumes 615 accumulates on the nozzle plate 102.

According to embodiments of the present invention, since solid sedimentdeposition is avoided by maintaining the nozzle plate damp, theaccumulated ink can be wiped off intermittently by a wiping serviceaction with an absorbent sponge or the like. Repeated rubbing eventuallydegrades the non-wetting coating of the nozzle plate. According to thepresent invention, since drying out of the ink on the nozzle plate isavoided, the time between successive mechanical wiping of the nozzleplate may be greatly increased relative to conventional systems thatrely upon wiping only without vapor control.

FIG. 8 illustrates tiny droplets forming on the printing head nozzlesurface according to an embodiment of the present invention. Thenon-wetting coating applied to the nozzle plate 102 prevents bigdroplets sticking to the nozzle plate surface. However, hot fumes 615emerging from the hot substrate printed image 660 form tiny droplets 670on the nozzle plate surface 102.

FIG. 9 illustrates large droplets that may disturb ejection of inkthrough the nozzles according to an embodiment of the present invention.The tiny droplets 670 illustrated in FIG. 8 may conglomerate into fewerlarger droplets 680 on the nozzle plate surface 102 and may distract theink jets and even block the nozzle plate nozzles with too high fumespressure.

FIG. 10 illustrates an ink droplet floating over tiny droplets accordingto an embodiment of the present invention. Experiments showed that whenthe nozzle plate is wet having tiny droplets 670, printing fluid thatcomes in contact with the nozzle plate in large droplets 690 does notleave traces of solid sediment on the nozzle. This can be explained inthat intimate touch of solid material in the ink with the nozzle plate'ssurface is prevented by the liquid layer of clear liquid that is spreadon the nozzle plate, which buffers between the solid particles (ordissolved material) and the nozzle plate. Intimate touch may also beprevented if the ink does not really touch the nozzle plate but ratherrides on the tiny droplets of clear liquid. The last sentence mayexplain the fact that even ink drops that stick on the nozzle plate donot leave solid sediment once they are removed. According to analternative hypothesis, ink droplets 690 that stick on the nozzle platedo not contaminate the nozzle plate because of a continuous supply ofspray or gas or steam of clear liquid also after an ink drop settles onthe nozzle plate, preventing the adhering of solids on the nozzle plate.The ratio of fumes being evacuated to fumes being accumulated on thenozzle plate 102 depends on the auxiliary suction system efficiency andcan be controlled by a skilled person. According to embodiments of thepresent invention a small amount of fumes on the nozzle plate preventboth sediment and printing liquid droplets accumulation as describeherein above.

FIG. 11 illustrates an evaporation bath as a source of fumes accordingto embodiments of the present invention. The fumes 615 may be generated,for example, by a warm pool of volatile liquid 665 underneath the headwhile the head exits the printing area 700. This option is particularlyrelevant in situations where printing is performed on a cold(non-heated) substrate such that the printing process does notinherently generate sufficient fumes to maintain the desired moisturelevel on the nozzle plate.

Wetting the nozzle plate may be required between each printing pass(e.g. each printed page). There could be cases, however, that it issufficient to wet the nozzle plate only once before or after purging, orbefore or after any other maintenance operation. In that case wetting isdone less frequently, as required. Another embodiment is spraying ordirecting liquid vapor or steam on the nozzle plate in-between printingruns (e.g. in-between different pages in 2D printing, or layers in 3Dprinting, or wafers in printing metal line on solar cells).

Assuming that the printing head comprises an array of nozzles, there aretwo ways to control the amount of fumes: evacuating part of the fumes byan auxiliary vacuum opening before the fumes reach the nozzle area andthe second way refers to a case where in head is shielded from thesubstrate's heat and fumes by a mask, wherein the mask comprising a slitthrough which the ink is dispensed on the substrate. In such cases mostof the fumes that enter the slit condensate at the center area of thenozzle plate as seen through the slit, and only minute fumes condensatenear the edge of the slit. Thus the head is located behind the mask suchthat the nozzles are close to one edge of the slit.

FIG. 12 illustrates an nozzle located close to the edge of the slitaccording to an embodiment of the present invention. The ink jets 715that have typically 20 um diameter are shown to pass close to the slitedge.

FIG. 13 illustrates a bottom view of the nozzle plate slit according toan embodiment of the present invention. Mask 720 has an opening in theform of a slit 730 and underneath the slit the nozzle plate 102 isillustrated. An array of nozzles 750 are illustrated wherein the slit730 is off center relative to the nozzles 750 such that a distance fromthe nozzles to one edge of the slit is less than 20% of a distance fromthe nozzles to the opposing edge of the slit. The condensed liquiddroplets 760 agglomerate mostly in the center of the slit 730 and only asmall fraction and smaller in diameter liquid droplets reach the slitedge where the array of nozzles 750 are located.

FIG. 14 illustrates a printing head assembly with an integratedself-purge and fumes removal arrangements according to an embodiment ofthe present invention. Auxiliary suction system 640 is deployed tomaintain a predefined level of moisture adjacent to the nozzle plate102. Auxiliary suction system 640 removes the major part of the fumes695 while smaller part of the fumes 615 reach the nozzle plate 102.Capillary gap 110 draws a large drop of purge printing liquid 510 whichmay be removed by vacuum pipe system (not shown). The suction systemprovides a pressure gradient to capillary gap 110, thereby removing thepurge printing liquid 510 from the capillary gap 110. The purge printingliquid 510 may be re-circulated to the printing head interior portionconfigured to contain printing liquid

The frequency of purging depends on the specific application for whichthe printing head is being used. In one embodiment, purging is initiatedperiodically as a step in a continuous printing process. In anotherembodiment, purging is initiated according to a predetermined schedule.In another embodiment, purging is initiated based on systemrequirements. One non-limiting example of system requirements is usingan inspection camera to track printing quality. If printing quality doesnot satisfy given parameters, purging is initiated.

An embodiment of the present invention facilitates an efficient printingsequence and purging maintenance cycle, taking advantage of purgingwithout dripping, and cleaning without a system element below theprinting mask. In one embodiment of a purging maintenance cycle, duringcontinuous printing a printing head stops printing, remaining in placeabove a printing surface. A purge is initiated that takes from 0.2 to0.5 seconds. After purging, a delay of about 5 seconds providessufficient time for substantially all purged liquid to flow from aportion of a nozzle plate that is aligned with a slit into the gapbetween the nozzle plate and a mask. Operation of a vacuum system for 1to 3 seconds provides pressure to suction the purged liquid from thegap. Printing can resume when suction is completed. Printing can alsoresume during suction, as described above, in which case suctionpressure should be verified to be an appropriate level to allow accurateprinting and accurate jetting direction. Printing resumes without theprinting head taking time to shift from the printing area to amaintenance area.

In an optional embodiment, if purging is not sufficient to clean theprinting nozzles and maintain printing quality, reduced pressure can beapplied near the nozzles from a source other than the printing head. Incontrast to applying positive pressure to the nozzles from inside theprinting head, applying pressure gradient can be more effective inopening a clogged nozzle. The system described in reference to FIG. 2can provide the required reduced pressure of about −0.4 to −0.8 Bar tofacilitate this method of cleaning the printing nozzles.

According to embodiments of the present invention, the printing systemmay be used in mass continuous printing, such as the case of printingmetal contact lines of solar cells above a conveyor system in a solarcell production line. In normal operation, the conveying system runscontinuously, requiring a printing system that runs continuously, orruns with minimal maintenance time. Normally the conveyor system is notable to be stopped to allow maintenance time for a printing system.Because a printing head in the printing system needs to purge, aconventional solution is to provide a redundant second printing head toreplace the active first printing head, allowing printing to continuewithout stopping the conveyor system using a separate purge area and amechanism to transfer the printing head to the purge area.

Using an embodiment of the present invention, depending on the specificsof the application, an active first printing head can perform a purgewithin the inherent small delay time between consecutive solar cells, orgroup of cells, with no need to shift the printing head to a maintenancearea and hence without slowing the continuous printing. The redundantsecond printing head can be activated to insure continuous printing, andthe previously active first printing head can be purged quickly usingembodiments of the present invention, facilitating the first printinghead being ready quickly to replace the second printing head,maintaining continuous printing with the required print quality.

According to embodiments of the present invention, the printing systemtypically includes a control system (not shown) including at least oneprocessor and configured by suitable hardware and/or software tocoordinate operation of the print heads, the various suction systems,the vapor control arrangement and/or the cooling system to perform thevarious functions described herein.

Advantageously, the printing head assembly of the present invention maybe used for printing solar cells grid-like contacts with typically 50 umline width and 20 um line height on a hot solar cells poly siliconsubstrate.

Another advantage of the printing head assembly described above is thatmass printing may be done continuously where purging the printing headassembly is performed during printing in the printing area with no needto shift the printing head to a maintenance area.

Another advantage of the printing head assembly described above is thatprinting metal contact lines may be done in one passage of the printinghead assembly over the substrate where the printing liquid evaporatesfast enough such that the next printing head ejects the printing liquidon a dry line of printed silver such that printed line builds up inlayers vertically and spread out horizontally on the hot surfaceminimally.

Furthermore, the present invention printing head assembly may be usedfor other ink jet printing applications such as ink jet printing onpaper. Any other application wherein the printing head needs efficientpurging also fall within the scope of the present invention.

In summary, the printing head assembly of the present invention improvethe prior art solar cells grid like contact printing ink jet heads byintroducing self purging, sediment prevention and fumes removalarrangements. The printing head assembly may enable faster and higherquality mass ink jet printing.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable sub-combination.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meanings as are commonly understood by one of ordinaryskill in the art to which this invention belongs. Although methodssimilar or equivalent to those described herein can be used in thepractice or testing of the present invention, suitable methods aredescribed herein.

All publications, patent applications, patents, and other referencesmentioned herein are incorporated by reference in their entirety. Incase of conflict, the patent specification, including definitions, willprevail. In addition, the materials, methods, and examples areillustrative only and not intended to be limiting.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather the scope of the present invention isdefined by the appended claims and includes both combinations andsub-combinations of the various features described hereinabove as wellas variations and modifications thereof, which would occur to personsskilled in the art upon reading the foregoing description.

What is claimed is:
 1. A printing head assembly with integrated purgemechanism, comprising: (a) a liquid dispensing head comprising one ormore dispensing nozzles enclosed in a nozzle plate, driven by at leastfirst and second pressures; (b) a shielding mask including an opening infront of said one or more nozzles; and (c) a capillary gap formedbetween said shielding mask and said nozzle plate wherein said openingis configured such that: (i) when printing liquid is dispensed from theliquid dispensing head driven by said first pressure, the liquid isdispensed through said opening in said shielding mask; and (ii) whensaid printing liquid is dispensed from the liquid dispensing head drivenby said second pressure, said printing liquid is drawn into saidcapillary gap, thereby removing said printing liquid from nearby saidnozzles.
 2. The printing head assembly of claim 1, comprising aplurality of printing heads, wherein said shielding mask comprising anopening in front of each of the printing heads.
 3. The printing headassembly of claim 1, comprising a cooling system configured to cool saidshielding mask.
 4. The printing head assembly of claim 1, furtherincluding a printing liquid storage system and a re-circulation systemthat returns said printing liquid that has been drawn to said capillarygap to said printing liquid storage system.
 5. The printing headassembly of claim 1, wherein said printing liquid is dispensed whilesaid printing head is in a printing area.
 6. The printing head assemblyof claim 1, wherein said shielding mask further includes an auxiliarysuction system deployed to evacuate at least part of the fumes thatemerge from the dispensed printing liquid on a heated printed substrate,or from a hot pool of volatile liquid underneath the printing head,while the printing head exits the printing area, to maintain apredefined level of moisture adjacent to said nozzles plate.
 7. Theprinting head assembly of claim 1, wherein said liquid is drawn to saidcapillary gap by a force selected from the group consisting of: a)capillary force; and b) applied vacuum suction.
 8. The printing headassembly of claim 1, wherein said shielding mask opening comprises aslit.
 9. The printing head assembly of claim 8, wherein said one or morenozzles are configured close to an edge of said slit such that saidprinting liquid is drawn to said capillary gap via said edge of saidslit.
 10. The printing head assembly of claim 1, further comprising asuction system in fluid interconnection with said capillary gap, saidsuction system providing: a) a pressure gradient to said capillary gap,thereby removing said printing liquid from said capillary gap; and b) adrawing force of the purge liquid towards said capillary gap.
 11. Amethod for printing comprising the steps of: (a) providing a printinghead assembly with integrated purge mechanism, each printing headassembly comprising: (i) a printing head including one or more nozzles;(ii) a shielding mask including an opening aligned with said one or morenozzles; and (iii) a capillary gap created between said printing headand said shielding mask; (b) printing by actuating said printing headwith a first pressure such that a printing liquid is dispensed from saidprinting head through said opening; and (c) purging by dispensing saidprinting liquid through said one or more nozzles with a second pressuresuch that the dispensed liquid is drawn into said capillary gap.
 12. Themethod of claim 11, wherein said liquid is drawn to said capillary gapby a force selected from the group consisting of: a) capillary force;and b) applied vacuum suction.
 13. The method of claim 11, wherein saidshielding mask opening comprises a slit.
 14. The method of claim 13,wherein said one or more nozzles are configured close to an edge of saidslit such that said printing liquid is drawn to said capillary gap viasaid edge of said slit.
 15. The method of claim 11, further comprising asuction system in fluid interconnection with said capillary gap, saidsuction system providing: a) a pressure gradient to said capillary gap,thereby removing said printing liquid from said capillary gap; and b) adrawing force of the purge liquid towards said capillary gap.
 16. Themethod of claim 11, further including the step of: d) re-circulatingsaid printing liquid that has been drawn to said capillary gap, to aprinting head assembly printing liquid storage.